Heat sink and its manufacturing method

ABSTRACT

Disclosed herein is a heat sink comprising a substrate of insulating ceramic and, superimposed on its upper surface, a diamond film for disposing an element thereon, which heat sink is excellent in heat radiation characteristics and can be stably used. This heat sink is obtained by providing a substrate of ceramic containing aluminum nitride of high thermal conductivity as a principal component; forming on the substrate a bonding member layer, such as a silicon film which is capable of being bonded with the substrate and a diamond film; and forming a polycrystalline diamond film of high quality on the bonding member layer so that the polycrystalline diamond film is strongly bonded with the substrate via the bonding member layer interposed therebetween in accordance with, for example, the vapor phase method.

TECHNICAL FIELD

[0001] The present invention relates to a heat sink and a process forproducing the same. More particularly, the present invention relates toa heat sink having such a laminate structure that a layer of diamond ofhigh thermal conductivity is superimposed on a substrate of aluminumnitride of high thermal conductivity, and further relates to a processfor producing the heat sink.

BACKGROUND ART

[0002] The processing capacity of electronic components is makingstriking enhancement in accordance with the increase of informationdensity. Thus, currently, a large amount of heat is emitted from eachelectronic component. Since maintaining constant temperature is desiredfor ensuring the stably functioning of electronic components, variousimprovements for cooling them have been proposed. It is common practiceto, in practical use, mount electronic components tending to have hightemperature on a heat radiating member, namely, “a material capable ofabsorbing heat, or a device including such a material for thermallyprotecting a constituent element or a system” known as a heat sink.

[0003] As materials having been in practical use in heat sinks for long,there can be mentioned metals and metal alloys of satisfactory thermalconductivity, such as Cu and Cu—W, and semiconductive or insulatingceramics of high thermal conductivity, such as SiC and AlN. However, asa result of integration of electronic components, the outgoing heat isincreasing to such an extent that the cooling with the use of a heatsink based on these materials only encounters a limit. Consequently, aheat sink material based on diamond exhibiting the highest thermalconductivity (about 2000 W/mK) among the existent materials, has beendeveloped as a new heat sink material for enhancing heat radiationcharacteristics. As such a heat sink which is now common, there can bementioned a substrate of copper and, brazed thereto, a single crystaldiamond in plate or film form, known as a submount. However, the heatradiation characteristics of this heat sink have not always beensatisfactory because the single crystal diamond is so expensive that aheat sink of large configuration cannot be employed and because thebrazing material becomes a resistor to heat conduction.

[0004] Therefore, forming of a polycrystalline diamond film on asubstrate according to the vapor phase synthetic process has been tried.For example, Japanese Patent Laid-open Publication No. 5(1993)-13843proposed a heat radiating member consisting of a substrate having asurface for mounting a semiconductor element thereon and, covering themounting surface, a diamond layer synthesized by the vapor phaseprocess. This heat radiating member is for the purpose of suppressingthe deterioration of performance of semiconductor laser elements bygenerated heat, and obtained by directly forming a polycrystallinediamond layer of 10 to 500 μm thickness on a substrate in accordancewith the microwave plasma CVD (Chemical Vapor Deposition) process. Itwould be feasible to use this heat radiating member as a submount byminiaturizing the same.

PROBLEM TO BE SOLVED BY THE INVENTION

[0005] In the above heat sink, the most suitable material of substrateon which a polycrystalline diamond film is to be formed varies dependingon the usage. Among various substrate materials, sintered aluminumnitride (AlN) is known as a material having not only insulatingproperties but also high thermal conductivity. The AlN substrate isespecially useful when it is intended to form a circuit on a substrate.Specifically, when it is intended to form a polycrystalline diamondlayer only on an area of substrate on which an element is to be mountedwhile forming a circuit on areas of substrate other than the area ofsubstrate on which an element is to be mounted, an insulating materialsuch as a ceramic is preferred as a substrate material from theviewpoint that additional formation of an insulating film is not needed.Moreover, a material of high thermal conductivity is desired foravoiding the drop of heat radiation efficiency, namely, maintaining highthermal conductivity with respect to the whole body of substrate.Therefore, the use of sintered aluminum nitride (AlN) as a substratematerial having not only insulating properties but also high thermalconductivity is to be considered.

[0006] In this connection, in the use of a metal of high thermalconductivity as a substrate, it is possible to obtain a substrate havingnot only insulating properties but also high thermal conductivity bylaminating the substrate with an insulating film of, for example, SiO₂in accordance with the vapor deposition method or the like. However,this insulating film often poses a problem with respect to thereliability in voltage withstanding properties, etc.

[0007] It is anticipated that the above heat sink comprising a substrateof sintered aluminum nitride (AlN) and, superimposed thereon, apolycrystalline diamond layer will have high availability.

[0008] However, it is difficult to directly form a polycrystallinediamond layer of high quality on a substrate of sintered aluminumnitride (AlN) in accordance with the vapor phase synthetic process. Fromthe practical viewpoint, any heat sink comprising a polycrystallinediamond layer of high thermal conductivity superimposed on a substrateof AlN is not known. For example, although the above Japanese PatentLaid-open Publication No. 5(1993)-13843 describes that Cu, Cu—W alloy,Cu—Mo alloy, Cu—W-Mo alloy, W, Mo, sintered SiC, sintered Si₃N₄,sintered AlN and the like can be used as substrate materials, thethermal conductivity of the polycrystalline diamond actually formed on asubstrate of sintered AlN is as extremely low as 300 W/m·K.

[0009] It is an object of the present invention to provide a method offorming a polycrystalline diamond layer of high quality on a ceramicsubstrate containing aluminum nitride (AlN) as a principal component. Itis another object of the present invention to provide a heat sink havingsuch a fundamental structure that a polycrystalline diamond film isformed on the above substrate, and having excellent heat radiationcharacteristics.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 is a sectional view of one representative form of heat sinkaccording to the present invention.

[0011]FIG. 2 is a schematic view of radiofrequency plasma CVD apparatuswhich can be appropriately used in the formation of a silicon film as abonding member layer in the production process of the present invention.

[0012]FIG. 3 is a schematic view of microwave CVD apparatus which can beappropriately used in the formation of a diamond film in the productionprocess of the present invention.

MEANS TO SOLVE THE PROBLEM

[0013] The inventors have made extensive and intensive studies with aview toward solving the above problem. As a result, it has been foundthat a polycrystalline diamond film of high quality exhibiting highthermal conductivity can be produced on a ceramic substrate containingaluminum nitride as a principal component by preliminary forming thereona layer of specified substance exhibiting excellent adhesion to thesubstrate and a diamond film and thereafter forming a polycrystallinediamond film on the above layer in accordance with the vapor phasemethod. The present invention has been completed on the basis of thisfinding.

[0014] Therefore, according to one aspect of the present invention,there is provided a heat sink comprising a ceramic substrate having atleast one plane containing aluminum nitride as a principal component,and a diamond film layer superimposed on the plane of the ceramicsubstrate, characterized in that the ceramic substrate and the diamondfilm layer are bonded together via a bonding member interposedtherebetween.

[0015] In the above heat sink of the present invention, a ceramicsubstrate containing, as a principal component, aluminum nitride whichis excellent in insulating characteristics and heat radiationcharacteristics is used as a substrate material. Accordingly, the heatabsorption performance and heat radiation performance of the heat sinkas a whole are excellent. Further, in case of the heat sink having sucha structure that the diamond film is formed so as not to cover theentire surface of the substrate, an electronic circuit made of a metalcan be printed on the substrate without the particular need to form aninsulating film.

[0016] In the heat sink of the present invention, it is preferred thatthe bonding member be constituted of at least one material selected fromthe group consisting of silicon, silicon carbide, tungsten, tungstencarbide, CuW, Cu—Mo alloy, Cu—Mo—W alloy, amorphous carbon, boronnitride, carbon nitride and titanium. In the use of this bonding member,cracking or other faults of the diamond film superimposed on the bondingmember can be avoided, the heat radiation performance is enhanced, andthe durability of the diamond film is prolonged.

[0017] This bonding member may be constituted of a crystalline substanceorientated on a specified crystal face. In particular, it is preferredthat the bonding member be constituted of a polycrystalline siliconpreferentially orientated for crystal face (111), crystal face (220) orcrystal face (400.) When the bonding member is constituted of acrystalline substance, the crystal grains of the polycrystalline diamondfilm formed on the bonding member would be enlarged and thecrystallinity thereof would be high, so that the diamond film would haveespecially high thermal conductivity.

[0018] According to necessity, the polycrystalline silicon may contain adopant. The addition of the dopant would enable enhancing theorientation of polycrystalline silicon and would also enable impartingelectrical conductivity to the silicon film.

[0019] In the heat sink of the present invention, preferably, thediamond film layer exhibits a thermal conductivity of 800 W/mK orgreater. The heat sink of the present invention attains enhanced thermalconductivity as compared with that of a heat sink wherein a diamond filmlayer is directly superimposed on a substrate.

[0020] According to another aspect of the present invention, there isprovided a process for producing a heat sink, comprising the steps ofproviding a ceramic substrate having at least one plane containingaluminum nitride as a principal component; forming a bonding memberlayer on at least portion of the plane of the ceramic substrate; andforming a diamond film on the bonding member layer.

[0021] In the above process of the present invention, peeling orcracking of the diamond film would not occur during the cooling stepafter the diamond film formation step wherein the substrate ismaintained at 700 to 1100° C. Thus, a heat sink of high qualityaccording to the present invention can be stably produced in highefficiency in the process of the present invention. In particular, whena layer of crystalline substance orientated for a specified crystal faceis formed as the bonding member layer, a diamond film of large crystalgrains and high crystallinity can be easily formed. Moreover, when alayer containing silicon as a principal component is formed as thebonding member layer, a diamond nucleation would be promoted, therebyenabling formation of a diamond film in higher efficiency. Furthermore,in the process wherein the bonding member layer is formed from aconductive substance, such as a polycrystalline silicon containing adopant, and wherein, while a direct current voltage is applied to thebonding member layer, a thin diamond film is formed on the bondingmember layer in accordance with microwave CVD or hot filament CVDmethod, it would be feasible to efficiently deposit an effectiveprecursor for diamond film synthesis on the substrate surface, therebyeasily forming a diamond film of high crystallinity, high orientationdegree and high thermal conductivity.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] The heat sink of the present invention is a laminate comprising aceramic substrate having at least one plane containing aluminum nitrideas a principal component, and, a diamond film layer superimposed on theplane of the ceramic substrate, characterized in that the ceramicsubstrate and the diamond film layer are bonded together via a bondingmember interposed therebetween. The terminology “heat sink” used hereinmeans a material capable of absorbing heat emitted by the aforementionedelements, or a device including such a material for thermally protectinga constituent element or a system. Thus, the terminology expresses aconcept comprehending what is known as a submount.

[0023] In practical use, various elements such as a semiconductorelement, a resistor and a capacitor are mounted on the diamond film.

[0024] The heat sink of the present invention includes a ceramicsubstrate containing, as a principal component, aluminum nitride whichis a material exhibiting high thermal conductivity despite havinginsulating properties. Therefore, even in the formation of a laminatewith the diamond film, the drop of total thermal conductivity can besuppressed. Further, when the diamond film is provided on not the entiresurface of substrate but part of the surface of substrate, a circuit forwiring can be simply printed on the part where no diamond film isprovided by the vapor deposition of a wiring material such as gold.

[0025] The above substrate for use in the present invention (hereinafteralso referred to simply as “AlN substrate”) is not particularly limitedas long as it is constituted of a ceramic containing aluminum nitride asa principal component and as long as its configuration is one having atleast one plane for formation of a diamond film on which an element ismounted. For example, the substrate may be a plate produced by adding asintering auxiliary to aluminum nitride powder, shaping the mixture, forexample, under pressure, and sintering thereof. Also, the substrate maybe one obtained by molding a polycrystalline aluminum nitride to a plateform.

[0026] The above diamond film may be constituted of a polycrystal or asingle crystal, and may be constituted of natural diamond or syntheticdiamond. From the viewpoint of cost and easiness of film formation,however, it is preferred that the diamond film be constituted of apolycrystalline diamond synthesized by the vapor phase method. The area,configuration, thickness, etc. of the diamond film are appropriatelydetermined depending on the mounted element, desired heat radiationcharacteristics, production time and production cost. Generally, thethicker the, diamond film, the greater the heat radiation. On the otherhand, the thinner the diamond film, the less the production time andcost. Therefore, taking a balance of these into account, it is preferredthat the thickness of the diamond film be in the range of 10 to 300 μm,especially 20 to 250 μm.

[0027] The higher the thermal conductivity of diamond film, the greaterthe advantage. In the employment of the production process of thepresent invention which will be described in detail later, hence, it ispreferred to use a polycrystalline diamond film exhibiting a thermalconductivity of 800 W/mK or greater, especially 1000 W/mK or greater, inthe heat sink of the present invention. In this connection, the diamondfilm is so thin that the thermal conductivity thereof cannot be directlymeasured. Therefore, the thermal conductivity of diamond film refers toa value calculated, after measuring of the thermal conductivity of aheat sink as a whole, from the thermal conductivity and thickness valuesof known materials constituting the substrate and the bonding member andthe measured thermal conductivity of the heat sink as a whole.

[0028] The most characteristic feature of the heat sink according to thepresent invention resides in that the AlN substrate and the diamond filmare bonded together via a bonding member layer interposed therebetween,wherein the bonding member has adherence to both the AlN substrate andthe diamond film. The interposition of the bonding member layer enablesproducing a diamond film of high quality without the peeling or crackingof diamond thin film during the production process. Further, after theproduction process, it enables avoiding the breakage or peeling ofdiamond film by a heating/cooling cycle during the use thereof.

[0029] This bonding member layer is not particularly limited as long asit is a layer constituted of a material having desirable adherence toboth the AlN substrate and the diamond film. However, from the viewpointof high adhesive strength (or bonding strength) to both the AlNsubstrate and the diamond film, it is preferred that the bonding memberlayer be constituted of at least one material selected from the groupconsisting of silicon, silicon carbide, tungsten, tungsten carbide, CuW,Cu—Mo alloy, Cu—Mo—W alloy, amorphous carbon, boron nitride, carbonnitride and titanium.

[0030] When the bonding member is constituted of a crystalline substanceorientated for a specified crystal face, the crystal grains of thepolycrystalline diamond film formed on the top of the bonding memberwould be enlarged and the crystallinity thereof would be high, so thatthe thermal conductivity of the diamond film would be enhanced.Accordingly, it is preferred that the bonding member be constituted ofsuch a crystalline material. Such a crystalline material can be, forexample, a polycrystalline silicon preferentially orientated for crystalface (111), crystal face (220) or crystal face (400); or apolycrystalline silicon containing a dopant, such as boron orphosphorus, preferentially orientated for the above crystal face. Thedoped orientated polycrystalline silicon is especially preferred fromthe viewpoint that, because it is conductive, a direct current voltagecan be applied at the forming of diamond film as described later.

[0031] Although the thickness of the bonding member layer is notparticularly limited, it is preferred that the thickness be in the rangeof 5 nm to 3 μm, especially 10 nm to 2 μm, from the viewpoint of abalance of bonding effect, effect on easiness in obtaining a diamondfilm of high quality, time taken in the formation of bonding memberlayer, drop of thermal conductivity by providing of the bonding memberlayer, etc.

[0032]FIG. 1 is a representative sectional view of heat sink “A”according to the present invention. The heat sink “A” has such astructure that aluminum nitride substrate 100 in plate form is laminatedwith bonding member 110 and diamond film 120 in this order. AlthoughFIG. 1 shows the form of heat sink wherein the entire upper surface ofthe aluminum nitride substrate 100 is covered by the bonding member 110and the diamond film 120, it is not always needed for the bonding memberlayer to adhere to the entire lower surface of the diamond film as theupper layer, and it is satisfactory that partial adherence be effected,as long as satisfactory bonding strength can be realized.

[0033] The process for producing a heat sink according to the presentinvention is not particularly limited. For example, the heat sink can beappropriately produced by forming the bonding member layer on at leastportion of the plane of the AlN substrate and forming the diamond filmon the bonding member layer so that the diamond film covers at leastportion of the bonding member layer. With respect to the bonding memberlayer formed in this production process, it is not particularly limitedas long as the formation is effected so that part or the entiretythereof is finally interposed between the AlN substrate and the diamondfilm. The formation of bonding member layer may be effected in theconfiguration of a film having an area larger than that of the diamondfilm, or a film having an area smaller than that of the diamond film.Moreover, the film-like configuration is not always needed. For example,the bonding member layer may be in the form of a lattice, or a pluralityof spots scattered at given intervals therebetween. It is preferred thatthe thickness of the bonding member layer be in the range of 5 nm to 3μm from the viewpoint of a balance between attained effects andproductivity.

[0034] In the formation of the bonding member layer, suitable methodsaccording to the material of the bonding member layer is appropriatelyselected from among the common film forming methods known as beinguseful in the formation of films on substrates without limitation.Examples of such film forming methods include printing, plating, vapordeposition, chemical vapor deposition (CVD), sputtering and laserabrasion methods. Of these, the vapor deposition and chemical vapordeposition methods enable forming a film of high purity with high filmthickness precision, thus providing especially effective means.

[0035] For example, the bonding member layer constituted of at least onematerial selected from the group consisting of silicon, silicon carbide,W, WC, CuW, Cu—Mo alloy, Cu—Mo—W alloy, titanium and BN can beappropriately formed by the vacuum vapor deposition method with the useof electron beams. In this method, a material consisting of the sametype of substance as that constituting the bonding member layer isplaced on a hearth in a vacuum chamber. This material is exposed toelectron beams, so that the material is melted and vaporized. The vaporof the material is deposited (vapor deposited) on a surface of substratedisposed in the vacuum chamber. Thus, the formation of the bondingmember layer is accomplished. At this stage, the thickness of depositedfilm can be accurately controlled by measuring the thickness ofsubstance deposited in film by means of a film thickness monitor using aquartz oscillator. At the vapor deposition, the temperature of substratemay be equal to room temperature, or the substrate may be heated.

[0036] When the bonding member layer can be formed by the CVD methodfrom a gaseous material of, for example, silicon, amorphous carbon,carbon nitride, titanium, silicon carbide or W, it is appropriate toemploy the CVD method. The formation of bonding member layer accordingto the chemical vapor deposition method can be appropriately performedby the use of parallel plate plasma CVD apparatus. In this method, a rawgas such as SiH₄ diluted with a diluent gas such as hydrogen gasaccording to necessity is introduced in a reaction vessel evacuated tovacuum. Radiofrequency power is applied to one of a pair of parallelplate electrodes arranged opposite to each other, thereby generatingradiofrequency gas plasma. Thus, a film of silicon, amorphous carbon,carbon nitride, titanium, silicon carbide or W can be formed on asubstrate disposed on the other electrode opposite to the aboveelectrode. The substrate is generally heated to about 50 to 500° C.although varied depending on film growing conditions. Measuring of thefilm forming speed for each forming condition in advance would enableaccurate estimation of a film thickness by controlling a film formationtime. As the diluent gas, use can be made of nondeposited gas such ashelium, nitrogen, argon, xenon, neon or krypton as well as hydrogen gas.

[0037] The shape of bonding member layer can be arbitrarily varied byetching after film formation or by masking the substrate at the time offilm formation.

[0038] In the production process of the present invention, the formationof bonding member layer is not only important for preventing the peelingor cracking of the diamond film layer formed on the bonding member layerbut also extremely important for enhancing the product quality of thediamond film formed.

[0039] Therefore, in one preferred mode of the present invention, thebonding member layer is constituted of a crystalline substanceorientated for a specified crystal face. The formation of bonding memberlayer from the crystalline substance orientated for a specified crystalface would enable maintaining the orientation of the sublayer at thetime of forming a polycrystalline diamond film thereon in accordancewith the vapor phase method, thereby attaining growth of apolycrystalline diamond film of high orientation. As a result, thediamond film of high crystallinity can be obtained.

[0040] In another preferred mode of the present invention, the bondingmember layer may be formed from a conductive substance. When the bondingmember layer is formed from a conductive substance, the bonding memberlayer is used as an electrode, and a direct current voltage is appliedto the bonding member layer at the time of forming the diamond film inaccordance with the vapor phase method. Consequently, an effectiveprecursor for diamond synthesis would be preferentially deposited on thebonding member layer, thereby enabling formation of a polycrystallinediamond film of large crystal grains with high crystallinity. Thecrystallinity increase and crystal grain enlargement of polycrystallinediamond film would enhance the thermal conductivity of the diamond film.As a result, a heat sink which is excellent in heat radiationcharacteristics can be obtained.

[0041] Although the method of forming the bonding member layer from thecrystalline substance orientated for a specified crystal face is notparticularly limited, when it is intended to form the bonding memberlayer whose principal component is silicon in accordance with the CVDmethod, the orientation of silicon film can be controlled by regulatingforming conditions. Thus, such a polycrystalline silicon film thatdiffraction peak assigned to crystal face (111), (220) or (400)preferentially appears when an X-ray diffractometry is conducted can beformed. For example, when it is intended to obtain a silicon filmpreferentially orientated on face (111) (when an X-ray diffractometry isconducted, the diffraction peak strength assigned to this face issignificantly greater than those assigned to other faces), filmformation is appropriately performed at high forming operationtemperatures. When it is intended to obtain a silicon filmpreferentially orientated on face (220) film formation is appropriatelyperformed under high reaction pressures. When it is intended to obtain asilicon film preferentially orientated on face (400), film formation isappropriately performed under such conditions that a silane halide gasand a silane hydride gas are mixed together at an appropriate ratio. Atthis stage, a mixture of reactive gas with a gasifiable compoundcontaining a dopant element selected from among the elements belongingto Group III or Group V of the periodic table, such as diborane orphosphine, can be subjected to film synthesis to thereby enablesynthesizing a conductive silicon film of P type or N type. When thebonding member layer is constituted of such a silicon film, both theabove orientation effect and voltage application effect can be exerted.Therefore, it is optimal to form, as the bonding member layer, apolycrystalline silicon film containing a dopant (namely, of P type or Ntype) which realizes preferential appearance of crystal face (111),(220) or (400) when an X-ray diffractometry is conducted.

[0042] In the production process of the present invention, the thusformed bonding member layer is overlaid with the diamond film. Asaforementioned, although the area, configuration, thickness, etc. offormed diamond film can appropriately be determined depending on themounted element, desired heat radiation characteristics, production timeand production cost, it is preferred that the thickness of the diamondfilm be in the range of 10 to 300 μm, especially 20 to 250 μm.

[0043] Although the method of forming the diamond film is notparticularly limited, it is preferred that film formation be effected inaccordance with the vapor phase method from the viewpoint that intendedfilm formation can be accomplished easily. As the vapor phase method,common vapor phase methods suitable for diamond film formation, such aschemical vapor deposition and laser abrasion methods, can be employedwithout limitation. Of these, the chemical vapor deposition method ismost appropriate since, among the current technologies, it can providemeans for stably producing a highly crystalline diamond film with highreproducibility. The chemical vapor deposition method can be classifiedinto techniques respectively utilizing radiofrequency wave, microwave, ahot filament, etc., according to production process characteristics. Ofthese, the technique utilizing microwave (microwave CVD method) and thetechnique utilizing a hot filament (hot filament CVD method) arepreferred. These productive techniques will be described below.

[0044] In these productive techniques, carbonaceous gasifiablesubstances including methane, acetylene, carbon dioxide and carbonmonoxide are generally used as the raw material of diamond film. Thesedeposited gases may be diluted with a nondeposited gas of, for example,hydrogen, oxygen, nitrogen, argon, xenon, neon or krypton.

[0045] Also, a gasifiable compound containing an element belonging toGroup III or Group V of the periodic table, such as diborane orphosphine, can be mixed with the above gases before diamond synthesis.When a diamond film synthesis is carried out in the presence of suchgases, a conductive diamond film of P type or N type can be synthesized.

[0046] The substrate temperature during the formation of diamond film,although not particularly limited, is preferably in the range of 600 to1200° C., still preferably 700 to 1100° C. When the substratetemperature is lower than 600° C., a diamond film containing amorphouscarbon in high proportion would be formed, so that the thermalconductivity would be low, thereby disenabling satisfactory exertion ofthe effects of the present invention. On the other hand, when thesubstrate temperature exceeds 1200° C., the bonding member layer maysuffer damages. Further, amorphous carbon maybe contained in the diamondlike the phenomenon occurring in the low temperature film formation.Therefore, the substrate temperature exceeding 1200° C. is unfavorable.Any method of heating the substrate can be employed without particularlimitation as long as the temperature can be set for the above ranges.For example, there can be mentioned a heating method wherein a heater isburied in a holder provided for mounting the substrate; a method whereinthe substrate is heated by radiofrequency induction heating; and amethod wherein, in the microwave plasma CVD method, the substrate isheated by microwave inputted for plasma generation. The pressure fordiamond synthesis is generally in the range of 0.1 mTorr to 300 Torr. Inparticular, in the microwave plasma CVD method, the pressure is in therange of 50 mTorr to 200 Torr. Further, in the microwave plasma CVDmethod, the output of plasma generation power source, althoughappropriately selected depending on the characteristics of formeddiamond film, is generally in the range of 300 W to 10 kW. The shape ofdiamond film can be arbitrarily varied by etching after film formationor by masking the substrate at the time of film formation.

[0047] With respect to the process for producing the heat sink “A” ofthe present invention as shown in FIG. 1 wherein the bonding memberlayer is constituted of silicon, particular modes thereof wherein use ismade of parallel plate type radiofrequency plasma CVD apparatus “B” asshown in FIG. 2 and also microwave plasma CVD apparatus “C” as shown inFIG. 3 will be described in greater detail below.

[0048] The apparatus “B” of FIG. 2 is a representative apparatus whichcan appropriately be used in the formation of bonding member layer 110.The apparatus “B” includes reaction vessel 201 constituted of, forexample, a stainless steel such as SUS 304, wherein vacuum ismaintained. The reaction vessel 201 is connected through exhaust ports203 a, 203 b, which are provided in a side wall of reaction chamber, toa vacuum source such as a vacuum pump, so that given vacuum ismaintained in the reaction vessel 201. In FIG. 2, numerals 205 and 207 adenote a turbo molecular pump and an oil rotary pump, respectively. Highvacuum can be attained in the reaction vessel by exhausting by means ofthese pumps. Numeral 206 denotes a mechanical booster pump, and numeral207 b denotes an oil rotary pump. These pumps are used at the time ofsynthesizing a silicon film. Further, vacuum valves 204 a, 204 b forregulating the exhaust rate are disposed. Still further, sample table202 a for mounting substrate 213 is disposed inside the reaction vessel201 of the apparatus “B”. Heater 214 for heating the substrate 213 isburied in this sample table so that there is provided means for enablingcontrol of the substrate temperature. This sample table is provided soas to pass through a bottom wall of the reaction vessel 201 and is soconstructed as to be vertically slidable by drive means not shown,thereby permitting position adjustment. Slide portion, not shown,between the sample table 202 a and the bottom wall of reaction vessel201 is fitted with a sealing or seal member for ensuring desired vacuumlevel in the reaction vessel 201. Radiofrequency application electrode202 b is disposed opposite to the sample table 202 a for mounting thesubstrate, so that radiofrequency wave from radiofrequency oscillator212 can be led into the reaction vessel 201 via tuning unit 211.Further, reactive gas supply ports 208 a, 208 b are disposed at an upperportion of the reaction vessel 201, so that gases can be introduced intothe reaction vessel 201 via reactive gas flow rate regulator 209.Radiofrequency gas plasma is generated between the parallel plateelectrodes (202 a-202 b) of the reaction vessel 201 by simultaneouslycarrying out feeding of reactive gas and application of radiofrequencywave, so that the reactive gas can be dissociated to thereby form thesilicon film on the substrate 213.

[0049] The apparatus “C” of FIG. 3 is a representative apparatus whichcan appropriately be used in the formation of diamond film 120. Theapparatus “C” includes reaction vessel 301 constituted of, for example,a stainless steel such as SUS 304, wherein vacuum is maintained. Thereaction vessel 301 is connected through exhaust ports 303 a, 303 b,which are provided in a side wall of reaction chamber, to a vacuumsource such as a vacuum pump, so that given vacuum is maintained in thereaction vessel 301. In FIG. 3, numerals 305 and 307 a denote a turbomolecular pump and an oil rotary pump, respectively. High vacuum can beattained in the reaction vessel 301 by exhausting by means of thesepumps. Numeral 306 denotes a mechanical booster pump, and numeral 307 bdenotes an oil rotary pump. These pumps are used at the time ofsynthesizing a diamond film. Further, vacuum valves 304 a, 304 b forregulating the exhaust rate are disposed. Still further, sample table302 for mounting substrate 313 is disposed inside the reaction vessel301 of the apparatus “C”. Heater 314 for heating the substrate 313 isburied in this sample table so that there is provided means for enablingcontrol of the substrate temperature. This sample table is provided soas to pass through a bottom wall of the reaction vessel 301 and is soconstructed as to be vertically slidable by drive means not shown,thereby permitting position adjustment. Slide portion, not shown,between the sample table 302 and the bottom wall of reaction vessel 301is fitted with a sealing or seal member for ensuring desired vacuumlevel in the reaction vessel 301. Microwave transmission window 315constituted of a dielectric substance, such as quartz or alumina, isdisposed at an upper portion of the reaction vessel 301, so thatmicrowave from microwave oscillator 312 can be led through tuning unit311 and propagated through a microwave guide tube into the reactionvessel 301. Further, reactive gas supply ports 308 a, 308 b are disposedat an upper portion of the reaction vessel 301, so that gases can beintroduced into the reaction vessel 301 via reactive gas flow rateregulator 309. Microwave gas plasma is generated above the substratemounting table of the reaction vessel 301 by simultaneously carrying outfeeding of reactive gas and application of microwave, so that thereactive gas can be dissociated to thereby form the diamond film on thesubstrate 313. At this stage, the diamond film can be formed whileapplying a voltage to the silicon film as the bonding member layer.

[0050] When it is intended to produce the heat sink “A” of the presentinvention, first, substrate 213 is set on substrate mounting part 202 ainside the apparatus “B”. The interior of reaction vessel 201 isexhausted to create vacuum. After exhausting until a vacuum of 5×10⁻⁶Torr or below is realized in the reaction vessel 201, not only is gaswhose flow rate has been regulated by the reactive gas flow rateregulator fed through the reactive gas supply ports into the reactionvessel 201, but also radiofrequency wave from the radiofrequencyoscillator 212 provided outside the reaction vessel 201 is transmittedthrough tuner 211 to thereby minimize reflection loss and inputted tothe radiofrequency application electrode 202 b. Consequently,radiofrequency gas plasma is generated, so that a silicon film is formedon the substrate 213. The formation of silicon film is carried out whilethe pressure inside the reaction vessel 201 is preferably in the rangeof 0.1 mTorr to 100 Torr, still preferably 50 mTorr to 50 Torr. Underthis pressure, a uniform and homogeneous silicon film of highcrystallinity can be efficiently formed. In the production process ofthe present invention, the substrate temperature during the formation ofsilicon film, although not particularly limited, is preferably in therange of 50 to 500° C., still preferably 80 to 350° C. In theradiofrequency plasma CVD method, although the output of plasmageneration power source is appropriately selected depending on thecharacteristics of formed silicon film, it is generally in the range of5 W to 2 kW. The radiofrequency oscillation frequency is preferably inthe range of 1 to 200 MHz, still preferably 5 to 150 MHz. However, theseconditions depend on the capacity and morphology of apparatus used inthe above synthesis and should not be determined generally.

[0051] Gasifiable substances containing silicon, such as SiH₄, Si₂H₆,SiHCl₃, SiH₂Cl₂, SiCl₄, SiF₄ and SiF₂H₂, are generally used as thereactive gas for deposition to be introduced in the reaction vessel 201.These deposited gases may be diluted with a nondeposited gas of, forexample, hydrogen, nitrogen, helium, argon, xenon, neon or krypton.

[0052] Also, a gasifiable compound containing an element belonging toGroup III or Group V of the periodic table, such as diborane orphosphine, can be mixed with the above gases before the synthesis ofsilicon film. When the synthesis of silicon film is carried out in thepresence of such gases, a conductive silicon film of P type or N typecan be synthesized.

[0053] Although the introduction rates of reactive gas and diluent gasare varied depending on production conditions, when it is intended toobtain a film of high crystallinity, it is generally preferred that thegases be introduced so that the total introduction rate falls within therange of 50 to 1000 cc/min. Further, although the ratio of reactive gasand diluent gas mixed is not particularly limited, the greater themixing ratio of diluent gas to reactive gas (diluent gas/reactive gas),the greater the tendency to formation of a silicon film of highcrystallinity. Also, the orientation of silicon film can be controlledby regulating the film formation temperature, reaction pressure and rawgas mixing ratio.

[0054] After the formation of the silicon film as the bonding memberlayer, the resultant substrate is taken out from the reaction vessel 201and set on substrate mounting table 302 inside the apparatus “C” fordiamond film formation. In the same manner as in the formation of thebonding member layer, the interior of reaction vessel 301 is exhaustedby a vacuum pump to create vacuum. In the same manner as in theformation of the bonding member layer, after exhausting until a pressureof 5×10⁻⁶ Torr or below is realized in the reaction vessel 301, not onlyis gas whose flow rate has been regulated by the reactive gas flow rateregulator fed through the reactive gas supply ports into the reactionvessel 301, but also microwave from the microwave power source 312provided outside the reaction vessel 301 is transmitted through tuner311 to thereby minimize reflection loss, led through the microwave guidetube 310 and inputted into the reaction vessel 301. Consequently,microwave gas plasma is generated above the substrate mounting table302, so that a diamond film is formed on the substrate 313 havingalready been provided with the bonding member. At the stage of diamondfilm formation, application of a direct current voltage to the siliconfilm as the bonding member layer for about an hour from the initialstage of film growth is especially effective in the forming of a diamondfilm of high crystallinity. The applied voltage is preferably in therange of +500 V to −500 V. The formation of diamond film is carried outwhile the pressure inside the reaction vessel 301 is preferably in therange of 0.1 mTorr to 300 Torr, still preferably 50 mTorr to 200 Torr.Under this pressure, a uniform and homogeneous diamond film of highcrystallinity can be efficiently formed. In the production process ofthe present invention, the substrate temperature during the formation ofdiamond film, although not particularly limited, is preferably in therange of 600 to 1200° C., still preferably 700 to 1100° C. In themicrowave plasma CVD method, although the output of plasma generationpower source is appropriately selected depending on the characteristicsof formed diamond film, it is generally in the range of 300 W to 10 kW.The microwave oscillation frequency is preferably in the range of 500MHz to 5 GHz, still preferably 1 GHz to 4 GHz. However, these conditionsdepend on the capacity and morphology of apparatus used in the abovesynthesis and should not be determined generally.

[0055] Carbonaceous gasifiable substances such as methane, acetylene,carbon dioxide and carbon monoxide are generally used as the reactivegas to be introduced in the reaction vessel 301. These deposited gasesmay be diluted with a nondeposited gas of, for example, hydrogen,nitrogen, helium, argon, xenon, neon, krypton or oxygen.

[0056] Also, a gasifiable compound containing an element belonging toGroup III or Group V of the periodic table, such as diborane orphosphine, can be mixed with the above gases before the synthesis ofdiamond film. When the synthesis of diamond film is carried out in thepresence of such gases, a conductive diamond film of P type or N typecan be synthesized.

[0057] Although the introduction rates of reactive gas and diluent gasare varied depending on production conditions, when it is intended toobtain a diamond film of high thermal conductivity, it is generallypreferred that the gases be introduced so that the total introductionrate falls within the range of 50 to 6000 cc/min. Further, although theratio of reactive gas and diluent gas mixed is not particularly limited,the greater the mixing ratio of diluent gas to reactive gas (diluentgas/reactive gas), the greater the tendency to formation of a diamondfilm of high crystallinity.

EXAMPLE

[0058] The present invention will be further illustrated below withreference to the following Examples, which, however, in no way limit thescope of the invention.

[0059] In the following Examples and Comparative Examples, each siliconfilm was formed with the use of apparatus of the structure shown in FIG.2, and each diamond film was formed with the use of apparatus of thestructure shown in FIG. 3. Moreover, in each of the following Examplesand Comparative Examples, the bonding member layer, diamond film andfinally obtained heat sink were evaluated according to the followingmethods (1) to (4).

[0060] (1) Measurement of Film Thickness

[0061] In the measuring of the thickness of bonding member layer, inadvance, a silicon or tungsten film is formed on a quartz substrate, andthe film thickness is measured with the use of a tracer type filmthickness meter. The measured thickness is divided by production time,thereby determining the film forming rate beforehand. The thickness ofparticular bonding member layer was calculated by multiplying the filmforming rate by the time taken in the formation of the particularbonding member layer. On the other hand, the thickness of diamond filmwas determined by observing a section configuration through a scanningelectron microscope.

[0062] (2) Orientation

[0063] The orientation of silicon film and diamond film was inspected byan X-ray diffractometry. With respect to the silicon film, the peaksassigned to faces (111), (220), (311) and (400) appear at 28.4° (2θ/°),47.3° (2θ/°), 56.1° (2θ/°) and 69.1° (2θ/°), respectively. On the otherhand, with respect to the diamond film, the peaks assigned to faces(111), (220), (311) and (400) appear at 43.95° (2θ/°), 75.40° (2θ/°),91.60° (2θ/°) and 119.7° (2θ/°), respectively. Accordingly, theorientations were evaluated by comparing the intensities thereof witheach other.

[0064] (3) Crystallinity

[0065] The crystallinity was evaluated by measuring the half-value widthof scattered waveform appearing at about 1333 cm⁻¹ in Raman scatteringspectroscopy. The smaller the half-value width or Full Width HalfMaximum (FWHM), the higher the crystallinity.

[0066] (4) Thermal Conductivity

[0067] The thermal conductivity was calculated by the formula:$\begin{matrix}{{{thermal}\quad {conductivity}\quad \left( {W\text{/}m\quad K} \right)} = {{density}\quad \left( {g\text{/}{cm}^{3}} \right) \times {specific}\quad {heat}\quad \left( {J\text{/}g\quad K} \right) \times}} \\{{{thermal}\quad {diffusivity}\quad \left( {{cm}^{2}\text{/}s} \right) \times}} \\{{100\quad {({constant}).}}}\end{matrix}$

[0068] Herein, the density was measured by the underwater densitymeasuring method, and the thermal diffusivity was determined by thenonlinear regression analysis according to the 2-dimensional ringmethod.

Example 1

[0069] Ceramic substrate (25 mm×25 mm×0.5 mm thickness) containingaluminum nitride as a principal component was set on the substratemounting table inside the radio frequency plasma CVD apparatus. Theinterior of the reaction vessel was exhausted to vacuum, andsimultaneously the substrate mounting table was heated to 120° C. Theapparatus was held undisturbed for about 30 min until the substratetemperature was stabilized, and the reduction of pressure within thereaction vessel to 5×10⁻⁶ Torr or below was confirmed. Monosilane gasand hydrogen were introduced in the reaction vessel at respective flowrates of 3 cc/min and 100 cc/min, and the internal pressure of thereaction vessel was set for 1.5 Torr by regulating the exhaust valve.Subsequently, radiofrequency wave from the radiofrequency power sourceat an output of 50 W was tuned by the tuner so as to minimize reflectionloss and inputted to the radiofrequency application electrode.Radiofrequency power was applied for about 2000 sec so that the siliconfilm of 100 nm thickness was deposited on the substrate. After thecompletion of deposition reaction, the residual gas inside the reactionvessel was drawn off, and, after the confirmation of the lowering ofsubstrate temperature to 100° C. or below, the reaction vessel wasopened to expose the same to the atmosphere. Then, the substrateoverlaid with the silicon film was taken out from the radiofrequencyplasma CVD apparatus. The orientation of the formed silicon film wasinspected, and no result of strong orientation on a specified face wasrecognized. Further, the thermal conductivity of the substrate havingthe silicon film deposited thereon was measured. The measurement showedthat the thermal conductivity was about 200 W/mK.

[0070] Thereafter, in order to superimpose the diamond film, theobtained substrate was set on the substrate mounting table inside themicrowave plasma CVD apparatus. The interior of the reaction vessel wasexhausted to vacuum, and simultaneously the substrate mounting table washeated to 1000° C. The apparatus was held undisturbed for about 1 hruntil the substrate temperature was stabilized, and the reduction ofpressure within the reaction vessel to 5×10 ⁻⁶ Torr or below wasconfirmed. Methane gas and hydrogen were introduced in the reactionvessel at respective flow rates of 12 cc/min and 300 cc/min, and theinternal pressure of the reaction vessel was set for 100 Torr byregulating the exhaust valve. Subsequently, microwave from the microwavepower source at an output of 5 kW was tuned by the tuner so as tominimize reflection loss, led through the window constituted of quartzand introduced in the reaction vessel. Microwave power was applied forabout 10 hr so that the diamond film of 50 μm thickness was deposited onthe substrate. After the completion of deposition reaction, the loweringof substrate temperature to 100° C. or below was confirmed, andthereafter the reaction vessel was opened to expose the same to theatmosphere. Then, the substrate overlaid with the silicon film and thediamond film was taken out from the microwave plasma CVD apparatus. Thediamond film on the substrate was visually inspected, and it was foundthat formation of the diamond film extended to edges of the substrateand that there was no film peeling. A section of the diamond film wasobserved through a microscope, and the thickness of the diamond film wasmeasured. It was found that the thickness was about 50 μm. Thereafter,the crystallinity of the diamond on the substrate was estimated by Ramanscattering spectroscopy. As a result, it was found that the FWHM of peakassigned to diamond structure, appearing at about 1333 cm⁻¹, was about8.5 cm⁻¹. Further, the orientation of the diamond layer was inspected,and, as found with respect to the bonding member layer, no result ofstrong orientation on a specified face was recognized. Still further,the thermal conductivity of the obtained laminate (heat sink) wasmeasured. The measurement showed that the thermal conductivity was about350 W/mK. Calculation from the thermal conductivity values showed thatthe thermal conductivity of the diamond film was 1850 W/mK.

Example 2

[0071] Heat sink was produced in the same manner as in Example 1 exceptthat the thickness of synthesized diamond film was 200 μm. The diamondfilm on the substrate was visually inspected, and it was found thatformation of the diamond film extended to edges of the substrate andthat there was no film peeling. A section of the diamond film wasobserved through a microscope, and the thickness of the diamond film wasmeasured. It was found that the thickness was about 200 μm. Thereafter,the crystallinity of the diamond on the substrate was inspected in thesame manner as in Example 1. As a result, it was found that the FWHM wasabout 6.8 cm⁻¹. Further, the orientation of the obtained diamond layerwas inspected, and no result of strong orientation on a specified facewas recognized. Still further, the thermal conductivity of the obtainedheat sink was measured. The measurement showed that the thermalconductivity after lamination with the diamond film was about 720 W/mKwhile that after lamination with the bonding member layer was about 200W/mK. Calculation from these values showed that the thermal conductivityof the diamond film was 2000 W/mK.

Example 3

[0072] Heat sink was produced in the same manner as in Example 1 exceptthat the bonding member layer was constituted of W. The diamond film onthe substrate was visually inspected, and it was found that formation ofthe diamond film extended to edges of the substrate and that there wasno film peeling. A section of the diamond film was observed through amicroscope, and the thickness of the diamond film was measured. It wasfound that the thickness was about 50 μm. Thereafter, the crystallinityof the diamond on the substrate was inspected. As a result, it was foundthat the FWHM was about 8.0 cm⁻¹. Further, the orientation of theobtained diamond layer was inspected, and no result of strongorientation on a specified face was recognized. Still further, thethermal conductivity of the obtained heat sink was measured. Themeasurement showed that-the thermal conductivity after lamination withthe diamond film was about 360 W/mK while that after lamination with thebonding member layer was about 210 W/mK. Calculation from these valuesshowed that the thermal conductivity of the diamond film was 1860 W/mK.

Comparative Example 1

[0073] Heat sink was produced in the same manner as in Example 1 exceptthat no bonding member layer was formed. The diamond film on thesubstrate was visually inspected, and film peeling was found in thevicinity of substrate edges.

Comparative Example 2

[0074] The same procedure for producing a heat sink as in Example 3 wasrepeated except that the material of the bonding member layer waschanged to nickel. However, a uniform diamond film of desired thicknesscould not be obtained. This showed that nickel was unsuitable to thebonding member.

Example 4

[0075] Silicon film of 100 nm thickness was deposited on the substratein the same manner as in Example 1 except that the substrate mountingtable was heated to 300° C. and that the output of radiofrequency powersource was 10 W. The orientation of obtained silicon film was inspectedin the same manner as in Example 1. It was found that the silicon filmhad orientation of (111). Further, the thermal conductivity of thesubstrate having the silicon film deposited thereon was inspected. Itwas found that the thermal conductivity was about 200 W/mK.

[0076] Thereafter, a diamond film of 50 μm thickness was formed on theabove substrate having the silicon film deposited thereon in the samemanner as in Example 1. Thus, a heat sink was obtained. Thecrystallinity of the diamond on the substrate was inspected. As aresult, it was found that the FWHM was about 7.9 cm⁻¹. Further, theorientation of the obtained diamond layer was inspected, and realizationof such a result that diffraction peak assigned to face (111) was largerthan the other peaks by about 2.5 times was recognized. Still further,the thermal conductivity of the obtained heat sink was measured. Thethermal conductivity was about 370 W/mK. Calculation from the thermalconductivity values showed that the thermal conductivity of the diamondfilm was 2070 W/mK.

Example 5

[0077] Heat sink was produced in the same manner as in Example 4 exceptthat, with respect to the conditions for forming the silicon film, thesubstrate was heated to 120° C. Various evaluations of the heat sinkwere effected. As a result, it was found that, with respect to thecrystallinity of the diamond on the substrate, the FWHM was about 7.7cm⁻¹. Further, with respect to both the silicon film and the diamondfilm, realization of such a result that diffraction peak assigned toface (220) was larger than the other peaks by about 4 times wasrecognized. Still further, the thermal conductivity values of thesubstrate having the silicon film deposited thereon and the finallyobtained heat sink were 200 W/mK and 380 W/mK, respectively. Calculationfrom these thermal conductivity values showed that the thermalconductivity of the diamond film was 2180 W/mK.

Example 6

[0078] Heat sink was produced under the same conditions as in Example 5except that diborane was fed at a flow rate of 5 cc/min at the stage offorming the bonding member layer and that a direct current voltage of−100 V was applied to the bonding member layer for 30 min in the initialstage of diamond film formation. Various evaluations of the heat sinkwere effected. As a result, it was found that, with respect to thecrystallinity of the diamond on the substrate, the FWHM was about 7.3cm⁻¹.Further, with respect to both the silicon film and the diamondfilm, realization of such a result that diffraction peak assigned toface (220) was larger than the other peaks by about 4 times wasrecognized. Still further, the thermal conductivity values of thesubstrate having the silicon film deposited thereon and the finallyobtained heat sink were 200 W/mK and 400 W/mK, respectively. Calculationfrom these thermal conductivity values showed that the thermalconductivity of the diamond film was 2400 W/mK.

EFFECT OF THE INVENTION

[0079] The heat sink of the present invention exhibits high heatradiation efficiency by virtue of the use of a ceramic substrateconstituted mainly of aluminum nitride of high thermal conductivity (AlNsubstrate) as a substrate on which a diamond film is formed. Further,the substrate is an insulator, so that it is practicable to cover partof the substrate with a diamond film while a circuit is formed on theother part of the substrate. Still further, in the heat sink of thepresent invention, the substrate and the diamond film are bondedtogether with satisfactory strength, so that, even if a heating/coolingheat cycle is repeated during the use thereof, the diamond film wouldnot suffer peeling or cracking. Therefore, the heat sink can be stablyused for a prolonged period of time.

[0080] Moreover, the production process of the present invention enablesefficiently producing the above heat sink of the present invention.Further, whilst the prior art of forming a diamond film directly on theAlN substrate in accordance with the vapor phase method has failed toform a diamond film of high quality, the production process of thepresent invention enables forming a diamond film of high quality on theAlN substrate, although indirectly, by virtue of the interposition of abonding member layer of specified substance.

1. A heat sink comprising a ceramic substrate having at least one planecontaining aluminum nitride as a principal component, and, a diamondfilm layer superimposed on the plane of the ceramic substrate,characterized in that the ceramic substrate and the diamond film layerare bonded together via a bonding member interposed therebetween.
 2. Theheat sink as claimed in claim 1, wherein the bonding member isconstituted of at least one material selected from the group consistingof silicon, silicon carbide, tungsten, tungsten carbide, CuW, Cu—Moalloy, Cu—Mo—W alloy, amorphous carbon, boron nitride, carbon nitrideand titanium.
 3. The heat sink as claimed in claim 1, wherein thebonding member is constituted of a crystalline substance orientated fora specified crystal face.
 4. The heat sink as claimed in claim 3,wherein the bonding member is constituted of a polycrystalline siliconorientated for crystal face (111), crystal face (220) or crystal face(400).
 5. The heat sink as claimed in claim 4, wherein thepolycrystalline silicon contains a dopant.
 6. The heat sink as claimedin any of claims 1 to 5, wherein the diamond film layer exhibits athermal conductivity of 800 W/mK or greater.
 7. A process for producinga heat sink, comprising the steps of: providing a ceramic substratehaving at least one plane containing aluminum nitride as a principalcomponent, forming a bonding member layer on at least portion of theplane of the ceramic substrate, and forming a diamond film on thebonding member layer.
 8. The process as claimed in claim 7, wherein thebonding member layer is formed from a conductive substance, and, while adirect current voltage is applied to the bonding member layer, a thindiamond film is formed on the bonding member layer in accordance withmicrowave CVD or hot filament CVD method.